1. Field of the Invention
The present invention relates to a thermoplastic resin soluble in aqueous alkaline solution and, more specifically, to a thermoplastic resin soluble in aqueous alkaline solution and comprising a terpolymer of isobutylene, maleic anhydride and n-butyl acrylate.
2. Description of the Related Art
Alkaline water-soluble thermoplastic resins have been used as raw materials for preparing aqueous adhesives, water-based paints, etc. Isobutylene-maleic anhydride copolymer is well known among such resins.
The isobutylene-maleic anhydride copolymer does not have substantial thermoplasticity because it has a very high melt viscosity and a comparatively low thermal degradation (decarbonation) initiating temperature. The copolymer therefore has been unable to be used for applications which utilize the meltability of the resin, such as hot-melt type pressure-sensitive adhesive and bonding adhesive and modifier for plastics and the like. Further the isobutylene-maleic anhydride copolymer has only limited applications because it is rigid and brittle and shows no substantial thermoplasticity and has poor processability.
To solve these problems, there has been proposed addition of a third copolymerizable component of monoethylenically unsaturated monomer, particularly an acrylic ester having a lower glass transition point, which improves the rigid and brittle properties of the isobutylene-maleic anhydride copolymer (U.S. Pat. No. 3,297,654, Japanese Patent Publication Nos. 7286/1965 and 20993/1969).
However, the methyl acrylate, ethyl acrylate and 2-ethylhexyl acrylate that are specifically illustrated in the above described U.S. Pat. No. 3,297,654, Japanese Patent Publication Nos. 7286/1965 and 20993/1969 have the following problems.
Methyl acrylate or ethyl acrylate does not have large capacity to soften (internal plasticization) the copolymer that forms upon its copolymerization with other monomers. It should there be copolymerized in a large amount for the purpose of improving the rigid and brittle properties of the isobutylene-maleic anhydride copolymer, which in turn decreases the solubility of the obtained copolymer in aqueous alkaline solutions to a large extent.
Although ethylhexyl acrylate has a satisfactory capability of softening the resulting copolymer, it has the problem of too low a reactivity upon copolymerization with isobutylene and maleic anhydride, thereby hardly yielding a copolymer having uniform composition.
Acrylic acid esters with the number of carbon atoms being similar to that of n-butyl acrylate, i.e. n-propyl acrylate, n-pentyl acrylate and n-hexyl acrylate, which are not specifically illustrated in the above U.S. Pat. No. 3,297,654, Japanese Patent Publication Nos. 7286/1965 or 20993/1969 though, are also unsuitable for the purpose of the present invention because of the following problems, in addition to their industrial unavailability.
N-propyl acrylate, which is an acrylic acid ester having a smaller number of carbon atoms than n-butyl acrylate, has no problem with respect to copolymerization reactivity with isobutylene and maleic anhydride, but is inferior in the improvement effect of internal plasticization and improves the thermal degradation initiating temperature only to a small extent.
N-pentyl acrylate and n-hexyl acrylate, which are acrylic acid esters having a larger number of carbon atoms than n-butyl acrylate, produces, like 2-ethylhexyl acrylate, a satisfactory effect as a softening constituent but is unsatisfactory in copolymerization reactivity with isobutylene and maleic anhydride, thereby producing the resulting copolymer in a low yield and rendering it difficult to obtain a copolymer having uniform composition.
An object of the present invention is to solve these problems and to provide an alkaline water-soluble thermoplastic resin being, while containing units from isobutylene and units from maleic anhydride, free from those drawbacks inherent to conventional isobutylene-maleic anhydride copolymers, i.e. low thermal degradation initiating temperature, poor heat processability and rigid and brittle features, and having superior film properties such as flexibility and tensile strength.